1. Field of the Invention
The present invention relates to an over voltage cut-off circuit . More particularly, the present invention to a cut-off circuit which cuts off the over voltage at the ear and mouth tie line.
2. Description of the Related Art
Private automatic branch exchanges are frequently connected together by a tie line. Such a tie line is different from a line through central office exchange between them. The tie line types include R/D (ring and down) type, B/W (both and way) type and E/M (ear and mouth) type. Among them, the E/M or "E & M" or "ear and mouth" type is widely used.
The tie lines are in demand because they are trouble free in connecting phone calls through office exchange. Therefore, the tie line network is used in cases that business phone calls are frequently between organizations in a company or corporation, of which the places of business are scattered over various areas.
The E & M (ear and mouth) tie lines are classified into two-line or four-line E & M (ear and mouth) types, depending upon whether Tip line or Ring line each is composed of one or two lines. The term "E line" comes from the word "ear." The term "M line" comes from the word "mouth." The E line refers to a line receiving a seizure signal from the tie line, the reception being from the other party. The M (mouth) line transmits a seizure acknowledge signal when receiving the seizure signal or tie line seizure signal. Also, the parts from the receiving and transmitting are typically respectively referred in the art as the "E" parts and the "M" parts. On this matter, exemplars of the contemporary practice include Schmidt et al. (U.S. Pat. No. 5,392,328, System And Method For Automatically Detecting Root Causes Of Switching Connection Failures In A Telephone Network, Feb. 21, 1995) shows a system and method for automatically detecting root causes of switching connection failures in a telephone network. An operating system is provided for an analog switching machine having switch fabric formed by multiple line and trunk line networks using interconnected switch stages, with each switch stage having multiple grid connections and making post-connection continuity checks. Korsky et al. (U.S. Pat. No. 4,894,860, Trunk Interface Circuitry For Telephone Systems, Jan. 16, 1990) discusses a trunk interface circuitry for telephone systems. More particularly, Korsky et al. discusses an improved trunk interface circuitry for telephone system common equipment, connecting a local customer system to a central office of the telephone company. Brown et al. (U.S. Pat. No. 4,516,070, Magnetic Current Sensor With Offset and Load Correction, May 7, 1985) discusses magnetic current sensor with offset and load correction. A mechanized system distributes the access, test and communication functions to the point of testing. Typically, the centralized switching facility serves the telephone loops and equipment to be tested. Stafford et al. (U.S. Pat. No. 4,467,148, Telephone Line Analyzer, Aug. 21, 1984) discusses a telephone line analyzer providing automatic attenuation measurement of tie trunk lines and other telephone lines through measurement of the electrical level of a tone responder. Formosa, Jr. (U.S. Pat. No. 4,398,064, Call-For-Service And Continuity Sensor Circuit, Aug. 9, 1983) discusses a call-for-service and continuity sensor circuit for use with a trunk circuit of a digital telephone system interfacing with a central office via ground start signaling to provide for detection of a "call-for-service" signal to the telephone system by the central office. Mehaffey (U.S. Pat. No. 4,310,725, Interface For Private Branch Exchange, Jan. 12, 1982) discusses an interface for private branch exchange. The interface connects a PBX line to a central office trunk and provides a supervisory signal for accepting hook switch and dial pulse inputs, which are being readily adaptable to ground start trunks and loop start trunks. Based on my study of these exemplars of the contemporary practice and art, I find that there is a need for an improved over voltage cut-off circuit.